3 Answers
3

Traditional unit Tiers were based on the Warcraft 2 model where you upgraded your main worker producing structure to get new units. Since both races had 2 possible upgrades, this lead to 3 tiers.

Starcraft 1 mimic'd this with the Zerg race but not all races. However, the terminology had stuck for how to describe various phases of the game. Tiers referred to how deep into the tech tree a player had gone and how much resources/time they had spent on tech. As a result a general terminology emerged, placing most units in a tier.

With the advent of Starcraft 2 and the addition of a number of earlier units, there has been a rise in referring to certain units as tier 1.5. These are units after tier 1, but still before tier 2.

Regular terminology is detailed as follows:

Tier 1

Zergling

Marine

Zealot

Tier 1.5

Roach, Baneling

Marauder, Reaper

Stalker, Sentry

Tier 2

Hydralisk, Infestor, Mutalisk, Corruptor, Overseer

Ghost, Hellion, Siege Tanks

Immortal, Phoenix, Void Ray, Observer, Warp Prism

Tier 2.5 (sometimes referred to as tier 2 or 3)

Banshee, Raven, Viking, Medivac

Tier 3

Ultralisk, Broodlord

Battlecruiser, Thor

Colossus, Mothership, Carrier, High Templar, Dark Templar

You'll note that tier 2.5 has only terran units, and is often just referred to as tier 2. This is true for all of 2.5 except for the Raven. Because the Raven requires so many upgrades to be effective, its sometimes referred to as Tier 3 and others as Tier 2. Its also worth noting there is some discrepancy around which tier ghosts should belong to.

These naming conventions are not absolute, but are common amongst professional casters (people who comment on professional games).

Essentially, a new "tier" is unlocked when a key building in the tech tree has been built. Units that can be built as soon as you have the first structure (such as marines, zealots, and zerglings) are tier 1. Stalkers are tier 1.5 because they also require a cybernetics core, and Void Rays are tier 2 because they require a Stargate.

As for how to tell what tier a unit is in, the only real way is to memorize the tech tree. The terminology is rather meaningless unless analyzing build orders and strategies; if you fluidly upgrade and build new buildings then you're not going to notice a big distinction.

EDIT: With Zerg it's easier to tell the tiers because you have to upgrade to a Lair and then a Hive which is much more concrete.

The problem with this rule is as follows: "Tier 1.5: Requires Barracks attached add on (sometimes Tier 1), or additional building" One could consider the Factor an additional building.
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tzenesJul 29 '10 at 21:44

This also puts Medivac and Viking in tier 3 which is not correct.
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StrixVaria♦Jul 29 '10 at 21:58

Viking is kinda a weird one, because in SC1 Starport = Tier 3, but by the time you get Vikings up in SC2 you're definitely in Tier 2. There isn't really good rules for SC2, only conventions and the established convention is Vikings as Tier 2.
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tzenesJul 29 '10 at 22:05

1

@tzenes @StrixVaria, The problem with TZenes post is that a Thor (tier 3?) can be out just as soon as any of his tier 2.5 units. If not sooner, and it techs up with the tier 2 units. It really belongs at tier 2.5
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McKayJul 29 '10 at 23:00

as I mentioned in my post tier 2.5 is more myth than anything else. They're units which are aligned usually either with Tier 2 or Tier 3. The thor's long build time, combined with armory and tech lab requirements usually place it in a game around Tier 3 timing. Remember, Tiers are decided on by convention, not a set of rules for organizing units.
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tzenesJul 29 '10 at 23:10